How many books are in the dark tower series?

There are 8 books in the Dark Tower series, but one of them is a prequel. Here's a breakdown:

Main Series:

1. The Gunslinger (1982)

2. The Drawing of the Three (1987)

3. The Waste Lands (1991)

4. Wizard and Glass (1997)

5. Wolves of the Calla (2003)

6. Song of Susannah (2004)

7. The Dark Tower (2004)

Prequel:

* The Wind Through the Keyhole (2012) - This book takes place between the events of "Wizard and Glass" and "Wolves of the Calla".

So, while there are technically 8 books, the main series is comprised of 7.